macOS 26.5 SDK: NSViewController gains isSelectable

A private stored let isSelectable: Bool in an NSViewController subclass compiles against the macOS 26.0 SDK (Command Line Tools), but the macOS 26.5 SDK in Xcode 26.6 fails with 'cannot override with a stored property isSelectable' and 'ambiguous use of isSelectable'. Rename the property (e.g. allowsTextSelection). Generally, Swift stored properties in AppKit subclasses can start clashing when a newer SDK adds a member of the same name, so typecheck against the same Xcode/SDK as CI. Pass the app's real -target (deployment target) so availability errors such as NSMenuItem.subtitle (macOS 14.4) also show up locally.